1. Field of the Invention
This invention relates to a magneto-optical recording medium capable of overwriting from which recorded information can be read out by the utilization of the magnetic Kerr effect and a recording method using the same medium.
2. Related Background Art
A magneto-optical memory is known as an erasable optical memory. The magneto-optical memory, as compared with a magnetic recording medium using a magnetic head, has the advantage that high-density recording and non-contact recording and reproduction are possible. However, to record information on such a magneto-optical memory, it has generally been necessary to erase (magnetize in one direction) recorded portions before recording.
So, there has been proposed a system in which a recording-reproducing head and an erasing head are provided discretely, or a system whereby recording is effected while a continuous laser beam is applied and at the same time, a magnetic field applied is modulated. However, the former recording system suffers from the disadvantage that the apparatus therefor is large-scaled and costly. The latter recording system suffers from a disadvantage which spoils the original merit of magneto-optical recording, such as high-speed modulation being impossible when a great magnetic field is applied, head crash being caused by the contact between the rotating medium and the magnetic head when the magnetic head is brought close to the surface of the medium for the purpose of high-speed modulation, or the magnetic head being readily affected by dust adhering to the surface of the medium.
So, there has been thought out a method of realizing overwriting by two kinds of recording, i.e., recording in which use is made of a vertically magnetized film of two-layer structure comprising a magnetic layer great in coercive force and low in curie temperature and a magnetic layer small in coercive force and high in curie temperature and a relatively great magnetic field is applied to the film to thereby uniformly magnetize only the layer small in coercive force in one direction and thereafter great laser power is applied to the film to thereby reverse the magnetization of this layer, or recording in which relatively small laser power is applied to the film and without the reversal of the magnetization of the layer small in coercive force and high in curie temperature being effected, the magnetization of the layer great in coercive force and low in curie temperature is uniformly magnetized in the direction of magnetization of the layer small in coercive force (U.S. Ser. No. 71,190).
However, in the above-described recording method using the conventional vertically magnetized film of two-layer structure, there is a recorded state in which a magnetic wall is present between the two magnetic layers and therefore, there is a problem in the stability of recording bits, and there is also the disadvantage that the magnetic field generating portion for uniformly magnetizing in one direction the layer small in coercive force becomes large-scaled.